The present invention is related to a fabrication process of a metal-semiconductor hydrogen sensor, and in particular, a fabrication process of a metal-semiconductor hydrogen sensor using an electroless plating method to form a metal electrode of the hydrogen sensor.
Due to the technology developments, modern industrial and medical applications use a large quantity of hydrogen as a raw material or other purposes. Hydrogen, however, is a flammable and explosive gas. When the concentration of leakage hydrogen reaches 4.65 vol % or more in air, a hazard of explosion will take place. Therefore, on considerations of industrial safety and environmental concern, hydrogen sensors are widely used in factories, laboratories and hospitals for accurately monitoring the concentration of leakage hydrogen. The large volume and high production cost are disadvantages of conventional hydrogen sensors. Besides, most of the sensors are passive elements, so that other additional equipment or a conversion circuit is required to perform the analysis or amplification. Therefore, the conventional hydrogen sensors can not become intelligent sensors. As a result, the development of a new and effective hydrogen sensor that is intelligent and of the active type has become an important topic in modern industries.
In recent years, due to the advance of silicon semiconductor technology, much attention has been attracted on the use of a Pd metal-oxide-semiconductor (MOS) structure as a semiconductor hydrogen sensor. The reason for using the Pd metal in the hydrogen sensor lies in that Pd has a good catalytic activity and can dissociate the hydrogen molecule adsorbed to the surface into hydrogen atoms. A portion of the hydrogen atoms diffuses through the Pd metal and is adsorbed to the interface between the metal and the oxide layer. These hydrogen atoms, after polarization, cause a change in the Schottky barrier height between the oxide layer and the silicon semiconductor and thus the electrical properties of the device. In the early days, I. Lundstrom proposed a Pd/SiO2/Si MOS field effect transistor structure with a Pd gate [Lundstrom, M. S. Shivaraman, and C. Svensson, J. Appl. Phys., 46, 3876 (1975)]. After the hydrogen being adsorbed to the Pd gate, the altered threshold voltage and terminal capacitance are used as the two bases for the detection of hydrogen. However, the use of a three-terminal device to realize the functions of a two-terminal device not only increases the cost, but also has increases process difficulties. Furthermore, the quality of the oxide layer will also influence the hydrogen detection capability. The quality of an oxide layer becomes unstable when the growth of the thin oxide layer is contaminated by ions. This results in the surface state pinning of Fermi-level of silicon semiconductor. Therefore, Schottky barrier height is less influenced by the polarized hydrogen atoms and subsequently the hydrogen sensitivity is lower. Many researches were focused on how to improve such a problem. For example, A. Dutta et al. used zinc oxide (ZnO) [A. Dutta, T. K. Chaudhuri, and S. Basu, Materials Science Engineering, B14, 31 (1992)] and L. Yadava et al. used titanium dioxide (TiO2) to replace the oxide layer of silicon dioxide [L. Yadava, R. Dwivedi, and S. K. Srivastava, Solid-St. Electron., 33, 1229 (1990)]. On the other hand, the use of a two-terminal type Schottky barrier diode seems to be a more intuitive approach. Without the unstable factors of the oxide layer, the sensitivity of the device to hydrogen has a significant improvement. Therefore, for example, M. C. Steelee et al. proposed a Pd/CdS structure [M. C. Steele and B. A. Maciver, Appl. Phys. Lett., 28, 687 (1976)], and K. Ito et al. proposed a Pd/ZnO structure [K. Ito, Surface Sci., 86, 345 (1982)]. The using II-VI compound semiconductor as the material is mainly due to the less effect of surface states of II-VI compound semiconductor as compared to the polarized hydrogen atoms.
Lechuga et al. (1991) prepared a hydrogen sensor of a Schottky barrier diode type on a substrate of II-V compound, wherein Pt metal was vacuum evaporated on a GaAs substrate [L. M. Lechuga, A. Calle, D. Golmayo, P. Tejedor and F. Briones, J Electrochem. Soc., 138, 159 (1991)]. They reported that a surface state pinning of Fermi-level of semiconductor occurred when the film was deposited by a high energy means, and thus the Schottky barrier height is less susceptible to be affected by polarized hydrogen atoms. These phenomena may be explained by the theory of DIGS model proposed by Hasegawa et al. [H. Hasegawa and H. Ohno, J. Vac. Sci. Technol., B5, 1130 (1986)].
In past years, wet method (also called solution method) was seldom adopted in a fabrication process of a semiconductor device, because its plating solution contains many chemical components and it involves complicated chemical reactions. However, the electroplating method gradually exhibits its advantages in the latter stages of the semiconductor fabrication process in view of its superior capabilities in planarization, step coverage, and the plugging required by fabricating the multi-level interconnects. In particular, electroless plating is easy to be carried out with lower cost and energy consumption, and is suitable to be adopted in a continuous process for industrial mass production.
A primary objective of the present invention is to provide a process for preparing a metal-semiconductor type hydrogen sensor.
Another objective of the present invention is to provide a process for preparing a metal-semiconductor type hydrogen sensor, wherein a metal electrode of the hydrogen sensor is formed by electroless plating technique.
The hydrogen sensor prepared according to the process of the present invention comprises:
a semiconductor substrate;
an n-type or p-type semiconductor film formed on said semiconductor substrate; and
an anode and a cathode formed on the same surface of said semiconductor film and isolated from each other, wherein a first metal as said cathode forms an Ohmic contact with said semiconductor film and a second metal as said anode forms a Schottky contact with said semiconductor film, wherein a thickness of said second metal and a material of which said second metal is made enable a Schottky barrier height of said Schottky contact to decrease when hydrogen contacts an exposed surface of said second metal.
In the present invention, the material and the thickness of said second metal electrode enable the hydrogen molecule to dissociate into hydrogen atoms when the hydrogen gas comes into contact with the exposed surface of said second metal electrode. Also, said hydrogen atoms diffuse through said second metal electrode, so said Schottky barrier height decreases.
A process for preparing a hydrogen sensor according to the present invention comprises the following steps:
a) forming an n-type or p-type semiconductor film on a semiconductor substrate;
b) forming a patterned first metal electrode on said semiconductor film, wherein said first metal electrode forms an Ohmic contact with said semiconductor film; and
c) forming a second metal electrode on said semiconductor film, said second metal electrode being isolated from said first metal electrode, wherein said second metal electrode forms a Schottky contact with said semiconductor film, wherein a thickness of said second metal electrode and a material of which said second metal electrode enables a Schottky barrier height of said Schottky contact to decrease when hydrogen gas is contacted with said second metal electrode.
Preferably, the process of the present invention further comprises thermal annealing said first metal electrode to enhance electric characteristics of said Ohmic contact after the formation of said first metal electrode in step b). More preferably, said thermal annealing is carried out at a temperature ranging from 300xc2x0 C. to 500xc2x0 C. for a period from 20 seconds to 5.0 minutes.
Preferably, step b) of the process of the present invention comprises the following sub-steps:
I. coating a photoresist layer on said semiconductor film;
II. imagewise exposing said photoresist layer with a photomask;
III. developing said imagewise exposed photoresist layer to transfer a pattern of said photomask to said photoresist layer, so that a patterned photoresist layer is formed, and thus said semiconductor film is partially exposed;
IV. depositing a first metal on the partially exposed semiconductor film; and
V. lifting-off said patterned photoresist layer to form said patterned first metal electrode on said semiconductor film.
Preferably, step c) of the process of the present invention comprises the following sub-steps:
i. coating a photoresist layer on a whole surface of said semiconductor film containing said first metal electrode;
ii. imagewise exposing said photoresist layer with a photomask;
iii. developing said imagewise exposed photoresist layer to transfer a pattern of said photomask to said photoresist layer, so that a patterned photoresist layer is formed, and thus said semiconductor film is partially exposed;
iv. depositing a second metal on the partially exposed semiconductor film; and
v. lifting-off said patterned photoresist layer to form said second metal electrode on said semiconductor film.
Preferably, said depositing in sub-step IV) of step b) and in sub-step iv) of step c) are carried out by physical vapor deposition such as vacuum evaporation.
Preferably, said second metal electrode is Pd, Pd alloy or Pt, and more preferably Pd. Said second metal electrode, preferably, has a thickness of 0.30 to 5 micron.
Preferably, said depositing in sub-step iv) of step c) is carried out by electroless plating technique. Said electroless plating, preferably, comprises contacting said partially exposed semiconductor film with a plating solution for a period of time, wherein said plating solution is an aqueous solution comprising metal ions of said second metal electrode, such as palladium ions, a complexing agent, a reducing agent, a pH buffer and a stabilizer. Said palladium ions are preferably provided by dissolving a palladium salt or palladium halide into water. Said complexing agent preferably is selected from the group consisting of ethylenediamine, tetramethylethylenediamine, ethylenediaminetetraacetic acid (EDTA) and N,N,Nxe2x80x2,Nxe2x80x2-tetrakis(2-hydroxypropyl)-ethylenediamine. Said reducing agent is preferably selected from the group consisting of hydrazine, hypophosphite, borohydride and formaldehyde. Said pH buffer is preferably boric acid or ammonia solution. Said electroless plating, preferably, comprises contacting said partially exposed semiconductor film with said plating solution having a pH value of 9-12 and a temperature of 20-70xc2x0 C. for a period of time ranging from 1 minute to 1 hour.
Preferably said electroless plating, prior to contacting said partially exposed semiconductor film with said plating solution, further comprises undergoing a sensitization treatment by contacting said partially exposed semiconductor film with a sensitizing solution, which is an acidic solution containing stannous ions, for a period of time, for example from 5 to 10 minutes; and subsequently undergoing an activation treatment by contacting said partially exposed semiconductor film with an activating solution, which is an acidic solution containing palladium ions, for a period of time, for example from 5 to 10 minutes.
Preferably, said semiconductor substrate is made of a semi-insulating InP or GaAs material.
Preferably, said semiconductor film formed in step a) of the process of the present invention is an n-type III-V compound, and more preferably, said n-type III-V compound has a doping concentration of 5xc3x971015 to 1xc3x971018 cmxe2x88x921. An appropriate thickness of said n-type III-V compound is 0.050 micron to 10 micron. Said n-type III-V compound can be n-type InP (n-InP) or n-type GaAs, and preferably is n-InP.
Preferably, said semiconductor film is formed by a metal organic chemical vapor deposition or molecular beam epitaxy deposition in step a) of the process of the present invention.
Preferably, said first metal electrode of the hydrogen sensor of the present invention is an AuGe alloy or AuGeNi alloy, and more preferably an AuGe alloy. Said AuGe alloy preferably has a thickness of 0.30 micron to 5 micron.
Preferably, said second metal electrode of the hydrogen sensor of the present invention has a C shape or a C-like shape, and said first metal electrode has a shape corresponding to the shape of said second metal electrode such that said first metal electrode is encompassed by said second metal electrode. Alternatively, said first metal electrode has a C shape or a C-like shape and said second metal electrode has a shape corresponding to the shape of said first metal electrode such that said second metal electrode is encompassed by said first metal electrode.
In order to further elaborate the objectives, characteristics and merits of the-present invention, preferred embodiments together with related figures are disclosed hereinafter.